We propose to examine the relative biological effectiveness of heavy ion radiation on cell lines grown as multicellular spheroids. The experiments have some properties associated with organized tissue systems. We will examine the mechanisms underlying the apparent fraction of radioresistant cells in spheroids. The role of cell cycle redistribution and anoxia will be evaluated and potential for repair of sublethal damage and potentially lethal damage will be measured, along with growth of intact spheroids as a function of dose and LET of heavy ion radiation. Alteration of cell cycle distribution in regrowing tumors will also be evaluated.